The effect of temperature on the conversion rates of volatile fatty acids (VFA) by thermophilic methanogenic sludge grown under different conditions was studied. Optimum temperatures for acetate degradation of sludges cultivated in serum bottles at 46, 55 and 64°C for 6–8 weeks were found to be strongly dependent on the cultivation temperature. However, sludges obtained after a start-up period of 6 months in Upflow Anaerobic Sludge Bed (UASB) reactors, fed with VFA mixtures at 46, 55 and 64°C showed comparable temperature optima, irrespective of the temperature of cultivation. A high temperature susceptibility for methane production and propionate degradation and, to a lesser extent for butyrate degradation, was found during the start-up of thermophilic UASB reactors. The reactors were started-up at 38°C with VFA mixtures. Thereafter, the process temperature was increased up to 55°C in steps of 5°C. Each increment led to a sharp drop in the methane production rate. However, no severe deterioration of methanogenesis was observed if the increase of the process temperature was performed very slowly between 50 and 55°C. The results indicate that, with respect to the application of thermophilic high-rate systems, the sensitivity to temperature fluctuations will decrease in time. A high sensitivity is expected if the maximum microbial growth-rate is the predominant selection criterion for the thermophilic methanogens.